I am looking for suggestions on how to evenly space packages on two metering belts. The system is already in the field and the belt speeds are controlled with VFDs which change speeds as needed. I am planning to use photoeyes to detect the packages as the enter the belts, but I need suggestion on how to maintain the spacing without using an encoder. The upstream section of conveyor prior to the metering belts accumulate packages back to back. As soon as the package enters the metering belts, the package needs to move at a specified speed to create the required space (12"). Any suggestion will be appreciated.

Run the second conveyor faster than the first. It should "pull a gap" between the packages as they transfer from one belt to the next. With some experimentation you should be able to get the speed ratio between the two belts where you want it.

Paul

Not an easy thing you ask, but I'll make an attempt to explain the math as I know it.
Assume two packages {Package A and Package B} each 20 inches long. First belt {Belt I} moves 10 inches per second, second belt {Belt II} moves 20 inches per second.
Start Time when Package A reaches End of Belt I.
0 to 1 second - Package A moves 10 inches into Belt II moving at Belt I Speed.
1 to 2 second - Package A moves at Belt II Speed meaning it moves 20 inches; Package B moves at Belt I speed and moves 10 inches ; meaning separation 10 inches.
2 to 3 second - Package A moves at Belt II Speed meaning it moves 20 inches; Package B moves at Belt I speed and moves 10 inches into Belt II; meaning separation 10 inches more for 20 inches total.
3 to 4 second - Package A moves at Belt II Speed meaning it moves 20 inches; Package B moves at Belt II speed meaning it moves 20 inches; meaning 20 inches total separation.

Now remember the accel from 10 to 20 ips is not instantaneous, but should be repeatable unless package coefficient of friction varies which it may as package weight and density varies. Next not all packages will be the same length.

To be fully accurate you'll need to run your accum conveyor at Speed I, Metering Belt I at Speed II and Metering Belt II at Speed III. Each belt will need to be faster than the one before. You'll need to use your PE's to calculate length when it passes from the Accum Belt to Meterning Belt I.

You can't achieve "perfect" speed control without an encoder. However, with sensorless flux vector drives, you can come to within a hair's breadth of achieving it. This should be good enough for your application. The rest of the suggestions explaining how to drive packages apart with two metering belts answers the details with one small detail. You may need to know the size of the package if they are different sizes if you want fixed spacing between packages (end to end), and not center-to-center. If you position your photo eye on the first belt such that it can measure the package size, then you should be able to do the calculation to know how fast to run the second belt, or even stop it if you need to wait on a package from the first belt.

And as I thought about this I realized you need to add another variable to the equation. Let's say you're using a PLC 5 and your scan time varies from 30 to 60 ms each scan.

At a line speed of 400 fpm or 6.6667 ips the 30 ms variation translates to 0.2 inches.

Using a Controllogix I'd strongly advise a periodic routine rather than a continuous scan unit.

Thank you paul. That is what we currently use in a section of conveyor, we move the belt faster to create a gap enough to transfer the package to a turner section.

Thank you for your reply. I will examine and test this logic. It should be achievable

Thank you for your suggestions. The system will run different package sizes for which we have the speed of each conveyor and PE's timer presets saved in a recipe type. Also, the spacing should not be perfect, but at least close enough. I forgot to mention that I have a robot arm after the metering belts. The robot laner will fill a third lane. The robot has PEs at the discharge end of the second metering belt to detect the leading edge of the package and then track it using its own encoder installed in their belt. The robotic laner developer wants to make sure we space the packages enough so the robot does not jam into orther lanes. Thanks againg for the suggestions.

This is easy with a motion controller.
1 You get the two belts to synchronized.
2. You measure the distance between product as described above.
3. You issue a phasing command to either the up stream or down stream belt to make the gap.
The phasing command assumes one of the belts is the master and the other the slave. The slave will then be told to super impose a move on top of the normal gearing that will either advance or retard the slave until the desired cap is achieved. These kinds of command usually required that you specify how you want to change the relative position of the slave belt as the master belt moves X distance. If there are 3 or four belts up stream and the master is the down stream belt then the phasing command must be issued to all down stream belts.

This was my first motion control project I had every done back in 1983 except I was synchronizing lug chains on a sorter and had to create gaps between pieces of wood.

The best way to accomplish this is by using a Metering wheel controlled by a VFD or DC speed controller depending on the type of motor you use. A Metering wheel gives you the timing to separate packages being transported in a conveyor. The gap between packages is control by the speed of the metering wheel.

G'day,
My suggestion is the same as Peter's, it's a pretty standard application and detailed (wth a pretty animation) here: http://www.ab.com/motion/applications/smartbelt_kinetix.html
Mike

Nice animation find Mike. Any idea how much AB charges for their cookie cutter code of this app?

Mechanical problems to be aware of before looking at controls
biggest problem with pulling gaps on different various sizes is the end roller size in relation to the smallest package. Small packages will bounce around if the end roller is too big of a diameter. Bouncing around will effect the photo eye if mounted close to end where the end roller, which will effect your gapping. When this situation happens usually the packages will be pushed together.

Otherwise its just either varying the speed in relation to how long the photo eye is blocked. Of course since you dont have an encoder gapping can not be precise.

Have you tried Metering wheels? I have five producction lines with 4 conveyors in each line all with metering wheels. Metering wheels allow you to control the gap between packages. Another solution would be using a gating system to separate the packages. Gating systems are control by photoswitchs and air.....More info. e-mail me at leonfdu@optonline.net.

Any chance you're allowed to post a picture or graphic of a metering wheel? Sounds interesting, but I cannot visualize.

Thanks :)

I'm not aware of a packaged motion code offering from RA but it might pay to ask them because it was a reasonably popular trade show demonstration and a standard exercise in the advanced motion course for field service and tech support. I did it in the course and it took me about three hours of programming time (after configuring the servos) to get it working nicely.

It's a slow rainy day so...

Definitions:
Lets call the conveyors from left to right Conv1, Conv2, Conv3 and Conv4, the Sensors Sens1 and Sens2
This exapmle is driven from the left conveyor.. you could similarly work everything out off the conveyor on the right if you need to synchronise with whatever is requiring the packages to be spaced.

Start-up logic:
Clear faults, home if required (Conv4) etc
Gear Conv2 to Conv1
Gear Conv3 to Conv1
Gear Conv4 to Conv1
Start up conveyor Conv1 (which starts the others moving)

Correction on Conv2
When sensor Sens1 sees something work out the difference between where the package edge is on Conv2 and where it should be to line up with flights on Conv4 assuming it travels at the speed of Conv1.
Issue an MAM on Conv2 to move forwards say 50% of that distance (Use the slowest acceleration you can to get the correction moves done within the length of the conveyor (less energy/slip and makes it look like a really effortless and professional job)).

Correction on Conv2
When sensor Sens2 sees something work out the difference between where the package edge is on Conv3 and where it should be to line up with flights on Conv4 assuming it travels at the speed of Conv1.
Issue an MAM on Conv2 to move forwards 100% of that distance (which will do the other half of the correction move).

Cautions:
Conveyor length, speed, acceleration, package width are important, make sure it works on paper before you try to build it.
If the package is light or slippery then the acceleration you can get away with on the moves is smaller (without doing things like putting a vacuum under a perforated conveyor, dual sided correction conveyor etc)
Use registration specific instructions (MAR, MAW) instructions rather than old school "IF Sensor1 THEN MAM(,,,,); END_IF", it will save having to worry about scan time and repeatability.
You may have to correct for slip or the changover between conveyors (small issue unless you're chasing mm's).
Make usre the flow rates work (you're not pushing in more packages than you have space for on the final conveyor).

Nice piece of code description Mike.

three hours, man that's fast. My first PLC 5 two to one merge took me 8 hours to fully iron out.